Re: Zetec to st170 spec?
I guess you would chose the ST170 over the Zetec for one major thing - insurance. Having gone through modifying a car to very basic levels (exhaust, rims, suspension) insurance companies take you for a ride, and sting for doing ANYTHING outside their guidelines.
But then I have special fleet discounts (about $4K off) for an ST170 so I can see your pain.
Using the nice motor the ST170 has you can strap a nice full exhaust and air filter keeping the mods simple and you have a real sleeper on your hands. If that gains (if your lucky) 20kw in total for a small outlay then you are looking at 147kw - turbo induction on a Zetec will leave little change from 10K, not to mention having to upgrade the motor's components to handle the forced induction (depending on how much boost you want to crank on to the motor), plus to be on the safe side upgrade the brakes, wheels/tyres and suspension to cope with the increase power.
So now you are already up to the ST170 level, without the trick interior, up to ST170 levels with expense and your insurer is going to point and skip around you and laugh quicker than you saying "full comprehensive insurance". Even a full exhaust on an ST170 will almost match turbo levels (not necessarily the torque, and that shove in your back, depending on how much you want to fry your front tyres!) but for a standard car with little effort you can have something pretty quick, and you can't exactly go screaming around the streets these days - you would be pretty happy with the outcome. And by the way you won't have as much stress, wear and tear, and maintenance with an NA motor as well. Factory turbo set ups are better engineered and have much more tolerance and are designed to cope with the stresses all in a standard setup. Thats why such things as the XR6 turbo are a great car to modify if you want the extra urge without the extra pain.
Well thats my opinion anyway, would be happy to hear what others have to think. This is just going from my research and experience so always willing to learn!!!